I can see the backtrace here.
Something is causing the full composer to re-render with the entire composer toolbar on every keypress.
The problem is that the way everything is glued answering “why” is very hard, typing is invalidating something.
Next step is to bisect to find the commit.
This is really really bad, we will need a new beta here @codinghorror
@eviltrout / @david / @j.jaffeux I will try bisecting this soon